pubs.acs.org/Langmuir © 2009 American Chemical Society
Fourier Transform Infrared Spectroscopy Study of CO Electro-oxidation on Pt(111) in Alkaline Media G. Garcı´ a, P. Rodrı´ guez, V. Rosca,† and M. T. M. Koper* Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, † Present address: Energy research Centre of The Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands Received June 23, 2009. Revised Manuscript Received August 14, 2009 In this work we investigate the electro-oxidation of CO on Pt(111) in alkaline solution by using Fourier transform infrared spectroscopy (FTIRS), to determine the adsorption sites of the CO, the intermediate species and the final oxidation product as a function of the applied potential. Multiple CO vibration bands (on-top, bridge and 3-fold hollow site) are observed on the Pt(111) electrode, their distribution and potential dependence being strongly dependent on the surface treatment. Spectroscopic results show that the final reaction product is carbonate and suggest that adsorbed carbonate blocks the access of CO from the (111) terrace to the active sites (i.e., step and kink sites).
1. Introduction The electrocatalytic oxidation of carbon monoxide is an important model reaction as well as a relevant practical reaction in fuel cells. It has been known for a long time that the anodic oxidation of CO occurs at a lower overpotential in alkaline media compared to acidic media, even on the pH-corrected RHE potential scale.1-3 Extensive studies have been carried out to understand the kinetics and mechanism of the CO oxidation on platinum electrodes in acidic media.4-8 Employing Pt[n(111)(110)] stepped electrodes, it was established that the CO oxidation reaction takes place preferentially at the most active oxidation sites, i.e, step sites, and that CO diffusion from the (111) terrace to these sites is very fast.6,9 Surprisingly, in a recent paper employing stepped Pt electrodes in alkaline media, four different active oxidation sites on the surface, i.e., sites with (111), (110), and (100) orientation, as well as kink sites were observed during CO stripping voltammetry, implying a low mobility of CO on the (111) terrace.3 Moreover, by a combination of chronoamperometry with voltammetry on a Pt(15 15 14) electrode in alkaline media, it was shown that there is a dual role of the step site toward the CO oxidation. CO adsorbed on one side of the step site is very reactive, while the CO located on other side *Corresponding author. Fax: þ31-71-5274451. E-mail: m.koper@chem. leidenuniv.nl. (1) Spendelow, J. S.; Lu, G. Q.; Kenis, P. J. A.; Wieckowski, A. J. Electroanal. Chem. 2004, 568, 215. (2) Markovic, N. M.; Lucas, C. A.; Rodes, A.; Stamenkovic, V.; Ross, P. N. Surf. Sci. Lett. 2002, 499, 149–158. (3) Garcı´ a, G.; Koper, M. T. M Phys. Chem. Chem. Phys. 2008, 10, 3802–3811. (4) Lebedeva, N. P.; Koper, M. T. M.; Herrero, E.; Feliu, J. M.; van Santen, R. A. J. Electroanal. Chem. 2000, 487, 37–44. (5) Strmcnik, D. S.; Tripkovic, D. V.; van der Vliet, D.; Chang, K.; Komanicky, V.; You, H.; Karapetrov, G.; Greeley, J. P.; Stamenkovic, V. R.; Markovic, N. M. J. Am. Chem. Soc. 2008, 130, 15332–15339. (6) Hoshi, N.; Tanizaki, M.; Koga, O.; Hori, Y. Chem. Phys. Lett. 2001, 336, 13– 18. (7) Vidal-Iglesias, F. J.; Solla-Gullon, J.; Campi~na, J. M.; Herrero, E.; Aldaz, A.; Feliu, J. M. Electrochim. Acta 2009, 54, 4459–4466. (8) Lopez-Cudero, A.; Cuesta, A.; Gutierrez, C. J. Electroanal. Chem. 2006, 586, 204. (9) (a) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen., R. A. J. Electroanal. Chem. 2002, 524-525, 242–251. (b) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. J. Phys.Chem. B 2002, 106, 12938–12947.
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(presumably adsorbed on top of the step) is less active and reacts with OH from the terrace.10 The CO electro-oxidation reaction is supposed to follow the Langmuir-Hinselwood-type mechanism first proposed by Gilman:11 H2 O T OHads þ Hþ þ e -
ð1Þ
COads þ OHads f COOHads
ð2Þ
COOHads f CO2 þ Hþ þ e -
ð3Þ
In strongly alkaline media the final product of the reaction (CO2) should convert quickly to carbonate. The formation of both surface adsorbed and dissolved carbonate can in principle be followed by Fourier transform infrared spectroscopy (FTIRS) employing different polarizations of the incident infrared radiation.12 The reasons for the apparent low CO mobility and the faster CO oxidation in alkaline solution compared to acidic media are still unclear. The aim of this paper is to employ in situ FTIRS to study CO adlayer stripping on a Pt(111) electrode in alkaline media. The influence of different surface preparations of the Pt(111) surface on the CO adsorption and oxidation will also be examined. These results will lead us to propose a model for the apparent low mobility of chemisorbed CO on Pt(111) in alkaline media.
2. Experimental Section Experiments were carried out in 0.1 M NaOH, prepared from high purity reagents (99.998%, Sigma-Aldrich) and ultrapure water (Millipore Milli-Q gradient A10 system, 18.2 MΩ cm, 2 ppb total organic carbon). Argon (N50) was used to deoxygenate all solutions and CO (N47) to dose CO. The working electrode was a Pt(111) single-crystal disk (0.785 cm2 geometric area) from Mateck. A Pt sheet was used as a counter electrode, and a reversible hydrogen electrode (RHE) was employed as the reference electrode. (10) Garcı´ a, G.; Koper, M. T. M. J. Am. Chem. Soc. 2009, 131(15), 5384–5385. (11) Gilman, S. J. Phys. Chem. 1964, 68, 70. (12) Arihara, K.; Kitamura, F.; Takeo; Tokuda, K. J. Electroanal. Chem. 2001, 510, 128–135.
Published on Web 09/24/2009
DOI: 10.1021/la902251z
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Electrochemical measurements were carried out with a computer-controlled Autolab PGSTAT12 potentiostat-galvanostat, while FTIRS experiments were performed with a Bruker Vertex80 V IR spectrophotometer equipped with a MCT detector. A spectroelectrochemical glass cell with a 60 °CaF2 prism was used, designed for the external reflection mode in a thin layer configuration. FTIR spectra were collected from an average of 20 scans obtained with 6 cm-1 resolution at selected potentials, by applying single potential steps from a reference potential (E0 = 0.05 V) in the positive-going direction up to 0.9 V. Spectra are represented as the ratio R/R0, where R and R0 are the reflectance measured at the sample and the reference potential, respectively. Consequently, positive and negative bands correspond to the loss and gain of species at the sample potential, respectively. S- and p-polarized light were used to distinguish between adsorbed and solution species: s-polarized light probes only species in solution whereas p-polarized light probes species on the electrode surface as well as in solution.12 Prior to experiment, the working electrode was annealed in a gas-oxygen flame, cooled in Ar þ H2 (3:1), and transferred to the IR cell (previously deaerated with Ar) after protecting the surface with a droplet of ultrapure water saturated with the cooling gases. Subsequently, in order to assess the quality of the system, the blank voltammetric profile of the Pt(111) surface in 0.1 M NaOH was recorded. Finally, the CO stripping experiments were obtained after bubbling CO through the cell for 2 min while keeping the Pt electrode at 0.05 V, followed by argon purging for 20 min to remove the excess CO and subsequently the electrode was pressed against the prism. For comparative purposes, we also carried out experiments with a Pt(111) electrode annealed in a Ar atmosphere, which typically leads to a higher number of defects in the surface.13-15
3. Results and Discussion 3.1. Electrochemistry of CO Stripping on Pt(111) Cooled in Ar and Ar þ H2 Gases. It has been documented extensively that the annealing and cooling down methodology for platinum electrode surfaces lead to a different quality on the surface structure.13-15 The presence of small structural changes in the surface play an important role in surface reactions, as was established for the CO stripping reaction in acidic media.15 In the case of the Pt(111) electrode cooled in a Ar þ H2 atmosphere or in a Ar-only atmosphere, no significant differences are observed in the blank voltammograms in 0.1 M NaOH (dotted line in Figure 1). Nevertheless, when a CO stripping is performed on these two differently prepared surfaces, the oxidation behavior is significantly different. Figure 1 shows the CO stripping voltammetry on Pt(111) annealed in Ar þ H2 gases for two scan rates (5 and 20 mV s-1) and on Pt(111) annealed in Ar atmosphere for 20 mV s-1. The anodic peak at higher potential (ca. 0.75 V) is related to the CO oxidation from the (111) terrace, while the peak at around 0.5 V is ascribed to the CO oxidation at defect sites (kinks, steps, etc.).3,10 A clear increase of the charge related to the CO oxidation at the defect sites is observed when the surface was cooled down in an Ar atmosphere. This result shows that different cooling procedures produce different surface structures. Pt(111) annealed in Ar results in a surface with a higher density of defects, while a Pt(111) electrode cooled down in Ar þ H2 atmosphere appears to give a much better defined (111) surface, even if some defects are still present. In agreement with previous work in acidic media,5,15 Figure 1 shows that the blank voltammogram is not (13) Clavilier, J.; Achi, K. El; Petit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 295, 333. (14) Kibler, L. A.; Cuesta, A.; Kleinert, M.; Kolb, D. M. J. Electroanal. Chem. 2000, 484, 73–82. (15) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. Electrochem. Commun. 2000, 2, 487–490.
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Figure 1. CO stripping voltammogram for Pt(111) in 0.1 M NaOH, for electrodes cooled down in H2/Ar (sweep rate: 20 and 5 mV s-1 for dashed and thin solid line, respectively) and in Ar (thick solid line, sweep rate: 20 mV s-1) atmospheres after annealing. Also included is the blank voltammogram for Pt(111) in 0.1 M NaOH, for an electrode annealed in Ar (dotted line, sweep rate: 20 mV s-1).
very sensitive toward detecting this small number of defects, but the effect on the catalytic activity is clearly very significant. Our CO stripping voltammetry for Pt(111) annealed in Ar is very similar to the CO stripping on Pt(111) reported by Spendelow et al. 1,16 These authors demonstrated that the peak observed at low potentials (0.4-0.6 V) corresponds to the CO oxidation at small Pt islands (diameter < 1 nm) on the Pt(111) surface, obtained after potential cycling at potentials higher than 0.90 V. Our earlier CO stripping voltammetry on stepped Pt electrodes in alkaline media also suggests that stripping peaks in this potential region are due to CO oxidation on step and kink sites in the (111) surface.3,10 3.2. Spectroscopic Results for CO/Pt(111) Cooled in Ar þ H2 Gases. Figure 2 shows a series of FTIR spectra obtained with p-polarized light during the oxidation of an adsorbed CO monolayer on Pt(111) in 0.1 M NaOH. The left panel was obtained with a reference potential at 0.9 V (R0.9), while the middle and right panels were obtained with a reference potential at 0.05 V (R0.05). When the R0.05 reference is used, two bands are observed at ca. 2040 and 1730 cm-1, which first appear as bipolar and turn into positive bands for potentials higher than 0.4 and 0.25 V, respectively. A bipolar band means that an adsorbed species is still present at the surface and undergoes an important wavenumber shift as a consequence of increasing the potential, whereas a positive band is related to a species observed at the reference potential but not at the sample potential. Furthermore, from ca. 0.2 V, a negative band is observed around 1808 cm-1 with its intensity increasing with more positive potential. These three bands are well-known in the literature17,18 and are associated to the stretching mode of CO adsorbed on top (COt ca. 2040 cm-1), bridge (COb ca. 1808 cm-1), and in a 3-fold hollow (16) Spendelow, J. S.; Goodpaster, J. D.; Kenis, P. J. A.; Wieckowski, A. J. Phys. Chem. B 2006, 110, 9545–9555. (17) Kunimatsu, K.; Shimazu, K.; Kita, H. J. Electroanal. Chem. 1988, 256, 371– 385. (18) Rodes, A.; Gomez, R.; Feliu, J. M.; Weaver, M. J. Langmuir 2000, 16, 811– 816.
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Figure 2. In situ FTIR spectra for the oxidation of CO adsorbed on Pt(111) in 0.1 M NaOH, for an electrode annealed in Ar/H2. The potential was stepped positively from 0.05 V in 0.05 V steps up to 0.9 V. CO was adsorbed at Ead = 0.05 V. Reference potential: 0.9 V for the left panel and 0.05 V for the middle and right panels. The sample potentials are indicated in each panel at the corresponding spectra.
(CO3h ca. 1732 cm-1) site. The latter can be observed clearly using the R0.9 reference, where the three CO adsorption states coexist in a wide potential range, and their bands appear as absolute due to the complete consumption of COads at the reference potential (0.9 V). From Figure 2 it can be observed that the COb feature is accompanied by the appearance of (initially) weak features at 1640 and 1400 cm-1. The former band is related to the O-H bending mode of water, due to the instability in the thin layer, while the latter band can be related to the production of a carbonaceous species. Interestingly, the spectra recorded with s-polarized light (not shown) show a negative band at 1640 cm-1 related to the water bending in the entire potential window,19 and a negative band at 1400 cm-1 only at potentials higher than 0.35 V, corresponding to the formation of carbonate in solution.20 Bicarbonate in solution also presents a symmetric stretching mode (νs) of the OCO at ca. 1400 cm-1 but the lack of a signal at 1650 cm-1 related to the asymmetric stretching mode (νas) of the OCO bicarbonate and the high pH of the working solution discard the existence of this species in our experiments.12,21,22 Also, this result was confirmed by performing the same experiments in deuterated water where the νas of the OCO group of bicarbonate was also absent (not shown). Free carbonate ions (19) Rasch, B.; Iwasita, T. Electrochim. Acta 1990, 35, 989–993. (20) Socrates, G. Infrared Characteristic Group Frequencies; John Wiley and Sons, Inc: Bristol, 1980. (21) Iwasita, T.; Rodes, A.; Pastor, E. J. Electroanal. Chem. 1995, 383, 181–189. (22) Freund, H.-J.; Roberts, M. W. Surf. Sci. Report 1996, 25, 225–273.
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have a planar structure, and only the doubly degenerate stretching mode near 1400 cm-1 is active in our frequency window. Therefore, it may be reasonably assumed that the signal at 1400 cm-1 at potentials higher than 0.35 V is associated to adsorbed carbonate as well as carbonate in solution, and at potentials lower than 0.35 V it is related to only an adsorbed carbonaceous species. However, we do point out that the absence of a signal recorded with s-polarization, does not necessarily mean that adsorbed species exists only on the surface, as was pointed out by Iwasita et al.21 Therefore, in order to discern between adsorbed and dissolved species, we carried out the following experiment using s-polarized light. The experiment entailed adsorbing CO at 0.05 V and subsequently stepping the potential to 0.25 V, after which several spectra were recorded with time. After ca. 2 min, a signal at ca. 1400 cm-1 related to dissolved carbonate started to appear. This shows that dissolved carbonate is produced at the same potential at which the COads oxidation takes place. Nevertheless, the longer time necessary to observe carbonate in solution compared to adsorbed carbonate has to be related to a slow desorption of carbonate from the surface into the solution or because dissolved carbonate is only formed after some minimal surface coverage of adsorbed carbonaceous species (i.e., carbonate) is reached. The signal around 1400 cm-1 recorded with p-polarized light is a rather broad band that makes it difficult to discern a band shift with potential for E > 0.35 V, also because this polarization observes both adsorbed and dissolved species. However, the absence of multiple signals in the spectra obtained with s-light in the high-wavenumber side of the band at E < 0.35 V and the appearance of a small negative hump around 1500 cm-1 at E > 0.6 V, which is observed with p-light but not with s-light, support the idea that this band corresponds to an adsorbed species, which is very sensitive to the substrate structure. Furthermore, as we discussed in section 3.1, for E < 0.6 V, the reaction takes place on defect sites (kink and step sites) and is not difficult to imagine that the reaction product adsorbs close to or in these special sites, resulting in the multiple IR signals referred to above. If we therefore assume the signal between 1500 and 1300 cm-1 in Figure 2 to be related to an adsorbed species (in addition to dissolved carbonate), two possible candidates could be suggested: (i) An adsorbed species such as Pt-COO- 22 or (ii) adsorbed carbonate.12,21 The first candidate is improbable as it is normally assumed to be a short-lived reaction intermediate in this potential region3 and is therefore unlikely to be observed by FTIRS. In addition, the signal related to OCO νs of this species (∼1660 cm-1) is not observed in the spectra, nor is it observed in the experiments carried out in D2O (not shown), implying that the ions are oriented at the interface with the molecular plane parallel to the electrode surface. Therefore, we assume that the signal around 1400 cm-1 has to be related to carbonate adsorbed in a perpendicular fashion to the platinum surface, considering that a carbonate absorbed flat on the negatively charged surface is unlikely.23 Identification of adsorbed carbonate is supported by studies of Iwasita and co-workers.21 They observed adsorbed carbonate binding through two and one oxygen on the Pt(111) electrode in acidic media. Arihara et al. observed monodentate carbonate on Au(111) in carbonate and bicarbonate solutions.12 Bidentate carbonate presents two fundamental modes in our frequency window, at ca. 1600 cm-1 related to the CO stretch of the noncoordinated oxygen atom and around 1280 cm-1 assigned to the asymmetric stretching of the coordinated oxygen atoms. Monodentate carbonate presents two fundamental modes related (23) Weaver, M. J. Langmuir 1998, 14, 3932.
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to the noncoordinated OCO group, i.e., νs between 1400 and 1500 cm-1 and νas at about 1350-1370 cm-1 .12,21,22 Therefore, the adsorbed species observed around 1400-1500 cm-1 in Figure 2 may be related to the OCO νs of singly coordinated carbonate, and the absence of the OCO νas suggests that carbonate is adsorbed perpendicularly to the surface. In this way, only the dipole moment of the symmetric stretching mode lies perpendicular to the electrode surface and therefore is the only one detectable by the surface selection rule. By plotting the intensity of the characteristic vibrational mode of each compound with the applied potential, it is possible to correlate them in order to draw up the different steps of the oxidation mechanism. Figure 3A displays the potential dependence of the integrated band intensities located at ca. 2035, 1808, and 1732 cm-1, which are associated to COt, COb and CO3h species, respectively. Figure 3B shows the potential dependence of the band intensity at 1400 cm-1 recorded with s- and p-polarized light corresponding to carbonate in solution and carbonate species both in solution and on the surface. In Figure 3, we can distinguish four different potential windows: E < 0.15 V. The intensities of COt, COb and CO3h are almost constant at E < 0.15 V, indicating the coexistence of these three adsorption states with no CO consumption in accordance with the stripping voltammogram where no faradaic current is observed in this region. A linear shift of the C-O stretch frequency with electrode potential is observed for COt, COb, and CO3h with Stark tuning slopes of 37, 81, and 61 cm-1 V-1, respectively. 0.15 < E < 0.3 V. In this potential window, we observe the onset of the first anodic peak in Figure 1, which is related to CO oxidation at defect sites (kink and step sites). In the same potential region, a slow decrease in the signal related to the COt and a slow increase in the signal at ca. 1400 cm-1 (p-polarized light), related to the carbonate formation, are observed. Also, the intensity of the CO3h band at 1732 cm-1 decreases sharply, accompanied by a fast rise in the COb signal at 1808 cm-1. A somewhat similar result was obtained by Markovic et al. 2 for CO oxidation on Pt(111) in the presence of dissolved CO in alkaline solution. Therefore, it is tempting to correlate the oxidation of COt near kink sites with the first stage of carbonate formation. This first small CO consumption produces a rearrangement of CO on the surface, resulting in an apparent CO3h conversion to COb. 0.3 < E < 0.6 V. An increase in carbonate production is observed with both s and p-polarized light in agreement with the first anodic peak developed in the stripping voltammogram recorded at 5 mV s-1. In this region of potential, the carbonate formation seems to stem from the oxidation of COt, as can be observed from the decrease of the signal at 2035 cm-1. The signal related to COb goes through a maximum at 0.35 V, where the carbonate production is already important. In this potential window, carbonate production seems to come from the reaction between OH adsorbed on defect sites and the most reactive CO, i.e., the single coordinated COt as well as some COb at higher potentials. 0.6 < E < 0.9 V. This potential window is related to the second anodic peak in the stripping voltammogram, which was previously ascribed to the CO oxidation with OH adsorbed on the (111) terrace.3 From Figure 3B a notable change in the carbonate production slope between 0.6 < E < 0.85 V is observable, while at E > 0.85 V a decrease of the signal is obtained, related to diffusion of soluble carbonate out off the thin layer. Figure 3A shows that only COb is still on the surface in this potential region, and COb is therefore the only candidate responsible for the carbonate production. This result is in agreement with density (24) Hammer, B.; Nielsen, O. H.; Noerskov, J. K. Catal. Lett. 1997, 46, 31.
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Figure 3. (A) Potential dependence of the integrated band intensity for COt (black circle), COb (square with cross) and CO3h (open triangle, intensity 3) obtained from Figure 1. (B) Potential dependence of the integrated intensity of the band corresponding to carbonate: p-polarized light (open hexagon), as obtained from Figure 2, and s-polarized light (black star, intensity 1.5), as obtained from spectra achieved with s-polarized light during the oxidation of CO adsorbed on Pt(111) in 0.1 M NaOH annealed in Ar/H2 (spectra with s-polarized light are not shown). Band intensities for adsorbed species were obtained by normalizing the spectra against a potential at which the species has been completely consumed such that an absolute band is obtained, minimizing overlap between neighboring bands.
functional theory (DFT) calculations of CO adsorption on stepped surfaces,24,25 in which it was found that CO adsorbed in a bridge site on the top of the step site is the most strongly adsorbed CO, and consequently the least reactive. This COb is oxidized by OH adsorbed on the Pt(111) terrace.10 It is important to comment on the intensities recorded with FTIRS (Figure 3) in relation to the faradaic current observed in the stripping voltammetry (Figure 1). In general, at lower potentials higher intensities are observed, while at higher potentials the intensities observed are lower than expected from the stripping voltammogram (Figure 1), both for the adsorbed and the dissolved species. For the adsorbed species, we must conclude that the IR absorption cross section of chemisorbed CO is much lower at low coverage. For dissolved species, the differences between stripping voltammetry and FTIRS experiments may be explained by the different techniques used: linear-sweep voltammetry in hanging meniscus mode, and a stepwise change in applied potential in a thin-layer configuration, respectively. Nevertheless, the two potential regions observed with higher and lower carbonate production slopes (Figure 3B) correlate well with the two oxidation peaks observed in the stripping voltammogram (Figure 1), and these two potential windows are related to the CO oxidation at defect (kink and step) and (111) terrace sites. However, it is important to realize that we must assume that Figure 3 gives only a qualitative idea of how the site occupation changes with potential. 3.3. Spectroscopic Results for CO/Pt(111) Cooled in Ar. In order to compare the FTIRS results obtained with the (25) Creighan, S. C.; Mukerji, R. J.; Bolina, A. S.; Lewis, D. W.; Brown, W. A. Catal. Lett. 2003, 88, 39.
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Figure 5. Potential dependence of the band center frequency for adsorbed carbonate as obtained from Figure 4.
Figure 4. In situ FTIR spectra for the oxidation of CO adsorbed on Pt(111) in 0.1 M NaOH for an electrode annealed in Ar gas. The potential was stepped from 0.05 V in 0.05 V increments up to 0.85 V. Adsorption potential was Ead = 0.05 V. Reference potential: 0.05 V. The sample potentials are indicated in each panel at the corresponding spectra. Inset: spectrum in the 1250-1600 wavenumber range, obtained by subtraction of the s-spectrum from the p-spectrum at 0.85 V, showing the band at ca. 1500 cm-1 attributed to adsorbed carbonate.
well-ordered Pt(111) electrode (Figure 2), infrared spectra of CO on a Pt(111) electrode annealed in Ar atmosphere were measured. The spectra in Figure 4 present two bipolar bands for E < 0.4 V; at higher potential, these bands become positive with band centers at ca. 2012 and 1785 cm-1, corresponding to CO adsorbed on top and bridge sites, respectively. Interestingly, the introduction of a small amount of defects dramatically changes the CO adsorption state as only COt and COb are observed on this surface and no CO3h. Apparently a long-range order of the (111) terrace is necessary to detect CO3h, as was reported for acidic media by Rodes et al.18 Also, a red-shift of the COt and COb band centers is observed, compared to the experiment carried out with the Pt(111) surface annealed in Ar þ H2. This band center shift may be attributed to the higher amount of defects on this surface.18 For E > 0.2 V, a new negative signal at ca. 1390 cm-1 is developed. This band corresponds to the formation of a carbonaceous species on the surface and in the thin layer, as was discussed in the previous section. Interestingly, with the introduction of small number of defects the band center at ca. 1400 cm-1 shifts with the applied potential, having a Stark tuning slope of 73 cm-1 V-1 for E < 0.45 V (Figure 5). The latter confirms the presence of a strongly adsorbed species. A potential dependence for this band center with a slope of 190 cm-1 V-1 was observed by Iwasita et al.21 for a Pt(111) electrode in acidic media, and by Arihara et al.12 on Au(111) in bicarbonate and carbonate solutions with slopes of 77 and 85 cm-1 V-1, respectively. Moreover, at E > 0.45 V, the negative hump at ca. 1500 cm-1 is more significant than for the well-defined (111) electrode, and its intensity increases apparently until 0.55 V, after which it remains constant. This band is not observed using s-polarized light (not shown). To intensify this band, we matched the intensities of the Langmuir 2009, 25(23), 13661–13666
Figure 6. Potential dependence of the integrated band intensity for COt (black circle), COb (crossed square), and carbonate (open hexagon) as obtained from Figure 4. Pt(111) electrode, annealed in Ar, in 0.1 M NaOH.
band at 1400 cm-1 at 0.85 V for both polarizations (s and p), and then subtracted the s-signal from the p-signal, so that a clear band is observed at ca. 1497 cm-1 (inset of Figure 4). This band, as we discussed above, was observed on Au(111) 12 and it was attributed to the OCO symmetric stretching of singly coordinated carbonate. The absence of the OCO asymmetric stretching indicates that carbonate is adsorbed perpendicularly to the surface (IR selection rule). This result confirms the existence of adsorbed carbonate through one oxygen atom to the surface, with its concentration apparently depending on the defect density, which could suggest that adsorption of carbonate take place close to defect sites (i.e., kink and step sites), since this is where the enhanced activity is located, although, strictly speaking, we cannot unequivocally ascribe the observed carbonate to being adsorbed at or near defects. Figure 6 shows the potential dependence of the integrated band intensities associated with COt, COb and CO32- species. The main differences with the experiment carried out on Pt(111) annealed in Ar þ H2 atmosphere can be summarized as follows: At E < 0.2 V only COt and COb are observed, with the intensity of COt increasing between 0.05 and 0.15 V, and the intensity of COb decreasing. The latter suggests a COb conversion to COt with more positive potentials. For E > 0.3 V, a fast decrease in the signal related to COt is observed, while the fast consumption of COb is observed for E > 0.4 V. For E > 0.55 V, only COb is present on the surface and as a consequence the only candidate to produce carbonate, in agreement with the observations on the DOI: 10.1021/la902251z
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Pt(111) cooled down in Ar þ H2. Two different slopes in the carbonate formation curve are observed (0.2 V < E < 0.6 and 0.6 V < E < 0.8 V), similarly to the results with the well-defined (111) electrode discussed in section 3.2. 3.4. General Discussion. The results discussed above clearly show that the flame annealing pretreatment is crucial in order to obtain a reproducible and well-ordered Pt(111) surface. Cooling down in a Ar þ H2 atmosphere, COt and CO3h are the main species adsorbed on the surface at low potentials. On the other hand, when the Pt(111) surface is annealed in Ar, defects (kink and step) are produced, and only COt and COb are observed by FTIRS. This latter observation emphasizes the importance of long-range order to obtain a compressed CO adlayer, as was discussed previously by Rodes et al.18 in their study of CO adsorption on Pt(111) and associated stepped surfaces in acidic media. Another important observation is the comparison of CO stretching frequencies on Pt(111) in acidic and alkaline media at low potentials, where the CO consumption is negligible. In acidic solution, the CO spectra show signals centered at 2068 and 1787 cm-1, corresponding to atop and 3-fold hollow CO, and their band center frequency shifts are 25 and 45 cm-1 V-1, respectively.18 On the other hand, our results in alkaline solution show CO adsorbed in three different states, and their stretching frequencies are all lower. The corresponding Stark tuning slopes for the COt and CO3h at low potential (E < 0.15 V) in alkaline media are 37 and 61 cm-1 V-1, respectively. Similar Stark tuning slopes were found in a previous study of CO oxidation on Pt(111) in the presence of dissolved CO in alkaline solution.2 Compared to acidic media, the “real” electrode potential is more negative (or the effective Fermi level more positive) in alkaline media, with respect to the pH-independent levels in solution (i.e., the NHE scale), and consequently an enhanced effect of back-donation into the Pt-CO bond may be expected.26 DFT calculations have shown that the latter effect may produce a stronger bonding interaction between Pt and CO, and that multifold coordination is favored at these negative electrochemical potentials.27-29 We argued in our previous paper3 that this effect may lead to an enhanced corrugation potential for CO on Pt(111), and that this might play a role in the observed lower CO surface mobility in alkaline solution compared to acidic solution. In spite of a presumably stronger CO bonding to Pt(111) in alkaline solution, this enhanced corrugation effect does not seem to be responsible for the apparent low CO mobility suggested in our previous CO stripping work.3 At low potentials, the FTIR experiments presented in this paper show three different adsorption sites (COt, COb and CO3h) with an apparently facile conversion between them. This implies a weak corrugation potential, and therefore a low barrier for CO hopping from one site to the next on the (111) surface, and as a result, a rapid CO diffusion from the (111) terrace to the most active site (kink and step sites) should be applicable also in alkaline media. Therefore, carbonate strongly adsorbed at or near defect sites appears as a more probable reason for the apparent blocking of the CO oxidation reaction. Summarizing, we suggest the following detailed mechanism for COads oxidation on Pt(111) in alkaline media: For E < 0.15 V, CO adsorbed atop and on 3-fold hollow sites are the principal species on the Pt(111) surface, and no faradic current is produced. (26) Blyholder, G. J. Phys. Chem. 1964, 68, 2772. (27) Koper, M. T. M.; van Santen, R. A. J. Electroanal. Chem. 1999, 476, 64. (28) Koper, M. T. M.; van Santen, R. A.; Wasileski, S. A.; Weaver, M. J. J. Chem. Phys. 2000, 113, 4392. (29) Curulla Ferre, D.; Niemantsverdriet, J. W. Electrochim. Acta 2008, 53, 2897–2906.
13666 DOI: 10.1021/la902251z
Garcı´a et al.
For 0.2 < E < 0.6 V, the first oxidation peak is developed in the CO stripping voltammogram, corresponding to the CO oxidation at defect sites (kink and step sites). FTIR spectra show the consumption of the COt (and CO3h) species in this potential window, with COb being the only adsorbate remaining on the surface for E > 0.6 V. In the same potential window carbonate (both dissolved and adsorbed) production is observed. Comparing the results on defect-poor Pt(111) with defect-rich Pt(111), the amount of monodentate carbonate appears to be dependent on the defect density, and therefore it likely adsorbs at or near these low coordinated Pt sites. The reaction in this potential window takes place at defect sites as a result of the preferential adsorption of OH. The product of this reaction (carbonate) adsorbs strongly at/near this active site, thereby blocking the CO oxidation reaction at least temporarily. We hold this effect responsible for the fact that four different active sites can be discerned in the stripping voltammetry on stepped electrodes,3 and emphasize that the present results give no evidence for a slow CO surface diffusion as we tentatively concluded in our previous paper.3 For E > 0.6 V, the second anodic peak is observed in the CO stripping voltammogram (Figure 1). Monodentate adsorbed carbonate obstructs the access of CO to the most active oxidation site. As a consequence, this stripping peak corresponds to CO oxidation on the Pt(111) terrace.3,10 FTIR suggests that COads is mainly bridge-bonded in this potential window, with a fair amount of COb probably binding to defect sites.
4. Conclusion The main spectral features of CO adsorbed on a Pt(111) electrode in alkaline media have been spectroscopically identified. Specifically, CO adsorbed on a 3-fold hollow site was observed for the first time in a CO-free alkaline solution study. Moreover, dissolved carbonate and singly coordinated carbonate adsorbed perpendicular to the surface were found to be the principal reaction products. It was also found that different annealing procedures produce different surfaces with different CO vibrational signatures and CO oxidation behavior. This stresses once more that appropriate annealing techniques should be used in order to minimize surface defects and to obtain a reproducible surface. Three different potential regions were identified during the CO oxidation on Pt(111) electrode in alkaline media. At E < 0.2 V, three CO states (COt, COb and CO3h) were observed simultaneously, with no reaction taking place. For 0.2 < E < 0.6 V, the CO oxidation commences and takes place exclusively on defect sites. The principal products are dissolved carbonate and monodentate carbonate, most likely adsorbed on or near the defect sites. It is proposed that the adsorbed carbonate blocks the active site, and consequently the CO reaction oxidation cannot take place there. Finally, at E > 0.6 V, OH adsorbs on the (111) surface, and the remaining CO, primarily adsorbed as COb, is oxidized. The FTIR results presented here do not support a slow CO diffusion on the (111) surface to the most active site, as suggested in our previous paper,3 but rather that the reaction product (carbonate) strongly adsorbs on the active site, thereby blocking it for further CO oxidation. As a consequence, several stripping peaks related to the CO oxidation at different sites (i.e., kink, step and (111) terrace sites) are observed during the CO stripping voltammogram.3 Acknowledgment. This work was funded by The Netherlands Organization for Scientific Research (NWO) through a “VICI” grant awarded to M.T.M.K., and by the European Commission through the FP7 Initial Training Network ‘‘ELCAT” (Grant Agreement No. 214936-2). Langmuir 2009, 25(23), 13661–13666
